"size" is the allocated size of the array, not the number of immediates
actually used. We could wind up returning a too-large constlen, larger
than 512, and since the binning variant uses the non-binning variant's
constlen as it's max_const we could make binning variants use c512.x and
crash when encoding.
Fixes: 86f3c0c4c2 ("ir3: simplify constlen calculation")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/40961>